Seat Formentor Diesel Engine Running Rich

Reduced engine power, rough running, and check engine light all point to fuel delivery problems. Is your fuel economy also down? When was the last time you changed your air filter? Has anyone checked the air-fuel ratio? I had similar issues on my Formentor, turned out to be the fuel injectors. Did you notice any jerking when accelerating?

Interesting. Fuel injectors, you say? How did you pinpoint that as the issue? Was it obvious, or did you have to do some diagnostic work?

Well, the symptoms were very similar to what you describe. I initially suspected a clogged air filter, but that wasn't it. Eventually, the engine started jerking when accelerating, which made me think it was fuel related. I didn't have the tools to properly diagnose the fuel injectors, so I took it to a workshop. They ran some tests and confirmed that one of the injectors was faulty. I'd suggest you do the same.

Took it to a workshop, and you were spot on. They confirmed a faulty fuel injector. Replaced it, and the car is running smoothly again. Total cost was 515€. Thanks for the help!